1. Technical Field
An embodiment of the present invention relates to a shock absorber including a damping-force generating device that controls a flow of oil occurring due to sliding of a piston in a cylinder and generates a damping force.
2. Related Art
For example, as a shock absorber used as a rear cushion that suspends a rear wheel of a motorcycle on a vehicle body, there is proposed a shock absorber including a cylinder in which the oil is filled, a piston slidably fit in the cylinder, and a piston rod coupled to the piston and extended to an outside of the cylinder. The shock absorber controls a flow of the oil due to the sliding of the piston in the cylinder and adjusts a damping force. As a shock absorber that changes a damping force, there is proposed a shock absorber including a solenoid actuator and configured to perform flow rate control for oil by changing an energization amount of the solenoid actuator.
In such a shock absorber, when a plunger that obtains thrust with a magnetic field generated in a solenoid operates, a plunger chamber is formed inside the solenoid to allow the plunger to operate. Fluid on the outside is guided to a fluid reservoir chamber, which is provided behind the plunger, via a through-hole formed in a rod operably coupled to the plunger. The plunger chamber and the fluid reservoir chamber are partitioned by a bearing. The fluid on the outside is guided to the fluid reservoir chamber via the through-hole of the rod rather than being guided to the plunger chamber. Therefore, the operation of the plunger is not hindered by fluid pressure on the outside, which improves responsiveness of the plunger (see, for example, Japanese Patent No. 4,667,609).
In the case of a shock absorber in which oil is used, a large amount of contaminants such as iron powder caused by friction of a valve and the like are included in the oil. Therefore, in the shock absorber having the configuration explained above, in some case, the contaminants intrude into the plunger that obtains the trust with the magnetic field generated in the solenoid, friction occurs, and a control characteristic is adversely affected. In particular, the contaminants tend to be attracted to and accumulate in a portion magnetized by a magnetic field generated in a coil. The problem due to the contaminants easily occurs.
For example, in the actuator described in Patent Literature 1 (Japanese Patent No. 4,667,609), the oil is prevented from directly intruding into the plunger chamber by the bearing. Even if the contaminants are included in the oil, the contaminants are prevented from intruding into a sliding section of the plunger. Even in such a configuration, the bearing section still cannot be a complete seal. In the bearing section where the rod is supported by the bearing, a flow of the oil occurs a little, for example, through a gap between the bearing and the rod. If the contaminants are included in the oil, the contaminants are pushed into the gap by the pressure of the oil. The contaminants are easily stuck. The plunger chamber in the actuator involves not only the pressure but also a strong magnetic field. Therefore, in particular, when contaminants of a magnetic material such as iron once occur, it is likely that the contaminants are held up and stuck.
In the shock absorber having the configuration, when so-called air entrainment in which air bubbles enter the plunger chamber occurs, during a rise of pressure, a flow of the oil corresponding to a volume change of the air bubbles generates an unintended flow of the oil. Consequently, during the rise of the pressure, harmful effects occur, for example, the actuator cannot withstand the pressure and performs unintended movement.